Lightweight aggregate

ABSTRACT

A lightweight aggregate for use in concrete mixes with Portland cement consists essentially of bottom ash produced as a by-product of burning pulverized coal in a dry-bottom furnace. The ash is sized to remove particles substantially exceeding 3/8 inch in diameter. It is substantially free of iron pyrites and sulfur trioxide to provide a lightweight aggregate which is inert, non-reactive, and meets the requirements for such aggregate as defined in ASTM specifications C-330, C-331 and C-332. Methods of producing this aggregate material are disclosed, including a method for recovering the same from the waste product ash in a power plant sluicing pond. Concrete mixes and products are disclosed in which such ash consists of the lightweight aggregate.

BACKGROUND OF THE INVENTION

This invention is directed to concrete products incorporating alightweight aggregate consisting primarily of bottom or base ashrecovered from the combustion of pulverized coal in a dry bottom typefurnace and to such bottom ash and to a method of producing the same foruse as a lightweight aggregate in concrete products.

A wide variety of materials has been used in the past as lightweightaggregates in the manufacture of concrete products. Such aggregates asare used in ready-mix concrete must meet ASTM:C330 specifications. Whenlightweight aggregates are used in hollow-load bearing concrete masonryunits, commonly known as concrete blocks, they must meet ASTM:C331specifications. When a lightweight aggregate is used in insulatingconcrete it must meet ASTM:C332 specifications.

Natural occurring materials which have been used as lightweightaggregates include pumice, scoria, and volcanic ash. Also, variousprocesses have been devised for expanding, calcining, or sintering suchproducts as furnace slag, clay, diatomite, perlite, shale, slate,vermiculite and fly ash. All of these products have been used in varyingextents as lightweight aggregate materials, and their use has dependedto a large extent on their relative availability as well as the cost ofhandling, preparation, and delivery. The expanding, calcining, orsintering procedures require the availability of energy in the form ofheat or steam, and particularly the calcining and sintering operationsrequire substantial quantities of heat at relatively high temperaturesto effect the modifications or change in the structure to make theproduct useful for masonry or concrete products, to eliminatedeleterious substances such as organic impurities, iron oxide, andunburned carbon. Examples in the patent literature of such substancesinclude:

U.S. Pat. No. 2,105,324: cinders, puffed slate, bloated or burned clayor shale, popcorn-slag, dry granulated slag.

U.S. Pat. No. 1,800,024: volcanic ash, ground cinders, fine groundpumice or sand.

U.S. Pat. No. 3,096,188: granular slag.

U.S. Pat. No. 3,362,837: expanded pumicite.

U.S. Pat. No. 3,245,812: ground oil shale ash.

Certain by-products of the burning of coal have been used widely aslightweight aggregate materials. One of the earliest coal products usedwas the clinker or cinder formed from burning lump coal on grates inbroiler furnaces or the like, as disclosed in U.S. Pat. Nos. 89,311 of1869, 145,277 of 1873, 642,301 of 1900, 664,710 of 1900, 770,083 of1904, and 991,419 of 1911. More recently the available cinder producthas been a by-product of burning crushed coal on travelling grates,under-fired grates or throwing stoker systems. Cinders, as a concreteaggregate, have fallen generally into disuse since the concreteproducts, particularly concrete blocks made with cinders, are unsuitablefor outside use due to their porosity, low compressive strength, andtheir tendency to stain. ASTM:C90 defines the specifications forconcrete block, and cinder blocks have been generally relegated to gradeS, that is, limited use above grade in exterior walls with weatherprotective coating and in walls not exposed to the weather.

In the 1920's a more effective method of firing power plant boilers cameinto use consisting of the pulverizing of the coal into a fine powder,the addition of a primary air, and the burning of the coal powdersubstantially in a suspended state within the furnace. Such coal ispulverized in a rolling mill to the point where approximately 70 to 80percent passes through a 200-mesh screen, is conveyed from thepulverizer with air into the furnace, and combustion takes place almostinstantly while the fine coal particles are in a suspended state. Thismethod of burning coal has come into wide use throughout the country dueto it increased efficiency of combustion. The ash content of the coal,which may vary from a low of about 4 percent to a high of about 20percent or more, is subject to the intense heat of combustion which mayrun between 2000° and 2800°F. Most of the ash is in the form of fly ash:the discrete sphere-like particles which are convected upwardly with theflue gases and are separated therefrom by electrostatic and/ormechanical collectors. The production of the large quantity of fly ashhas presented substantial disposal problems to the public utilitiessince only a small part of this material is commercially used. Theremaining or unutilized fly ash is frequently carried by water into alarge holding or sluicing pond where the material settles and may laterbe removed by dump trucks to a land fill. Due to the nature of the flyash particles, it has found use both as a pozzolan and as a lightweightaggregate. In the latter form it has been pelletized, sintered, and thencrushed and screened to provide proper gradation and is a good aggregatematerial.

However, the manufacture of a lightweight aggregate from fly ashrequires the employment of energy as well as machinery for pelletizing,or extruding, drying, and then firing on a sintering grate atapproximately 2200°F. The resultant product is similar in nature toexpanded shale and clay. U.S. patents which deal with the use of fly ashalone or in combination in concrete products and with the treatment offly ash for its use in concrete products include the following:

U.S. Pat. No. 2,250,107 of 1941: fly ash used in its natural state inlarge quantities as a substitute for cement. Examples are also givenwhere cinders were added as coarse aggregates.

U.S. Pat. No. 2,987,408 of 1961: forming a pozzolanic material using aspecific magnetic separation to remove magnetic particles.

U.S. Pat. No. 3,030,222 of 1962: fly ash nodulized with a binder ofsewage sludge and fired.

U.S. Pat. No. 3,192,060 of 1965: combining fly ash with alkaline earthreactants, pelletizing, and curing to form a lightweight aggregate.

U.S. Pat. No. 3,328,180 of 1967: pelletizing fly ash, drying thepellets, firing the pellets to an ignition temperature, cooling,crushing and refiring at a temperature between 1000° and 2000°F.

U.S. Pat. No. 3,669,701 of 1972: an oil well cement uses hollowcenospheres (float ash) as a lightweight aggregate.

U.S. Pat. No. 3,759,703 of 1973: mixing fly ash with calcium carbonate,firing and heating to a temperature of at least 1500°C to form a moltenslag, water quenching, and grinding to obtain a hydraulic binder.

U.S. Pat. No. 3,782,985 of 1974: using fly ash cenospheres as alightweight aggregate.

Boux, "Canadians Pioneer New Fly Ash Processing System", MineralsProcessing, March 1969, (pp. 16-19).

When coal is burned in pulverized form in a furnace, such as that usedin the generation of steam at power plants, a major portion of the ashcontent of the coal is expelled as fly ash while a minor portion of theash content, which may run as high as 40 percent, for some types of coalor furnaces, down to as low as 10 percent with other coals or furnaces,collects on the furnace walls or falls or runs to the furnace bottom andit is collected and removed from the furnaces by various arrangements.Two types of pulverized coal burning furnaces have come into general useand include the slag or wet bottom furnace on the one hand and the drybottom furnace on the other hand. Each produces a distinctivelydifferent bottom ash.

In the slag or wet bottom cyclone furnace, the ash content which iscollected at the bottom of the furnace is maintained in a liquidcondition by maintaining a temperature on the slag well above its fusiontemperature. Such slag temperatures may be from 1900° - 3000°F. A slagtank or water filled pit is positioned below the furnace to receive thetappings of molten slag from the furnace.

The resulting product has become known in the industry as wet bottomboiler slag or bottom slag. It is glassy and angular in appearance,resembling angular crushed dark-colored glass. Boiler slag has foundextensive use in asphalt paving mixtures and has been promoted as anaggregate for surface de-slicking in bituminous concrete. It has alsobeen used as a convention aggregate in the manufacture of concreteblocks and as a sandblasting material. It is usually relatively heavy inweight, exceeding 65-70 lbs. per cubic ft., and for this and otherreasons has not been suitable as a qualifying lightweight aggregate asdefined by relevant ASTM specifications.

The dry bottom furnace, formed with a hopper bottom, has a sufficientcooling surface so that the ash which impinges on the furnace walls oron the hopper bottom is solid and is essentially dry. These furnacesgenerally have an open grate at their bases and below the open gratethere is generally provided a water-filled ash pit to receive the ashesfrom the furnace.

A certain amount of molten slag will also form on the internal walls ofthe boiler and will find its way into the ash pit. However, a largeportion of the dry bottom ash is collected in an essentially dry stateand thus has physical characteristics which distinguish it from wetbottom slag. This is due to the fact that during the combustion ofpulverized coal, some of the molten ash particles are heavier and fallto the bottom where they cool and are collected dry in the form ofdiscrete spheres. Other smaller molten ash particles tend to agglomerateand fuse together and thus form discrete lumps or masses which becometoo heavy to be entrained or captured in the flow of combustion gasesthrough the furnace, and these larger particles also fall to the bottom.In the wet bottom furnace, they simply fall into the slag pool and losetheir individual characteristics. However, in the dry bottom furnace,they generally retain their spherical or agglomerated-sphericalidentity. Thus, while dry bottom ash may have in it some angular partswhich have a porous surface, a substantial portion of the dry bottom ashhas the appearance of a fine sand and when examined under magnification,the spherical nature of these particles may be observed and it can benoted that they appear essentially to be internally porous rather thanexternally porous, and the predominant material is light in color with asandpaper-like surface texture.

The utilization of both wet bottom slags and dry bottom ashes,particularly in the art of highway construction, as an aggregate incement stabilized road bases, is described in Seals et al, "Bottom Ash:An Engineering Material", Journal of Soil Mechanics and FoundationsDivisions, Proceedings of the Society of Civil Engineering, April, 1972,pp. 311-325; Moulton et al, "Utilization of Ash from Coal Burning PowerPlants in Highway Construction", reprint from Highway Research Record430 by the National Ash Association Inc. (undated); Moulton, "Bottom Ashand Boiler Slag, Ash Utilization Proceedings", Third International AshUtilization Symposium, U.S. Bureau of Mines Information Circular 8640,pp. 148-169, 1973; and Blocker et al, "Marketing Power Plant Aggregatesas a Road Base Material", U.S. Bureau of Mines Information Circular8640, pp. 208-233.

In the dry bottom furnace, the ash is collected, as noted above, in anessentially dry state and is dumped or collected in a water-filled ashpit. From there it is carried by water from time to time or continuouslyinto a sluicing pond. Frequently a pair of crusher rollers at the ashpit crush the larger chunks of ash down to a size approximately 11/2inch maximum. In the sluicing pond, the ash is eventually drained of itswater content as the pond fills, and the solids may be removed andhauled in dump trucks to a land fill.

During the burning of the coal, some of the ash tends to accumulate onthe surface of the boiler tubes. This material is known to be rich insulfur trioxide. From time to time, the tube ash is removed by water orsteam jets or by acid bath, where this material falls to the collectionapparatus at the bottom of the furnace and is then sluiced out into thepond with the bottom ashes.

SUMMARY OF THE INVENTION

I have found that by properly selecting and/or processing dry bottomash, it may be employed as the basic lightweight aggregate in themanufacture of concrete masonry units, structural concrete, andinsulating concrete and fully meets the strict requirements of thecorresponding ASTM specifications. It consists of inorganic materialsincluding iron oxide, silicon dioxide, alumina, magnesium oxide,titanium dioxide, and calcium oxide. It may also contain some sodium,and potassium oxides, chloride, sulfate, and phosphorous as acidicconstituents. Thus this bottom ash forms the basic ingredients of thelightweight aggregate of this invention. My invention centers around thediscovery of certain properly collected and/or selected dry bottom asheswhich fully meet the requirements of these specifications for alightweight aggregate for concrete mixtures and in fact may be so usedwith a minimum of follow-up handling or processing. The process does notrequire the application or energy in the form of heat as does theprocessing of fly ash or certain other aggregate materials. Statedanother way, my invention centers around the recognition of the possiblepresence of certain deleterious substances found in pond ash from drybottom furnaces and to methods of collecting and processing dry bottomash and pond ash from dry bottom furnaces to assure a product free ofthese deleterious substances and which is inert and otherwise fullymeets the applicable specifications.

The usual dry bottom furnaces installed at electrical power plants andsimilar utility plants produces three by-products which are usuallyconsidered as waste: (1) an excess of fly ash, which may be from 50-90percent of the ash content of the coal, (2) bottom ash, and (3) ironpyrites and other trash elements in the coal. Since these are commonlyconsidered by the power plants as waste products, they are frequentlybut not always, sluiced or dumped together into the pond along with theintermittently dumped boiler tube slag as previously described.

An excessive fly ash constituent in the pond is deleterious to alightweight aggregate as it increases the mass of the product and itpresents too high a proportion of fines to meet the specifications.

The presence of the sulfur trioxide is deleterious as it causes anundesired reaction with lime (calcium hydroxide) to form calciumsulfate. This effect is known as "aggregate growth" and the internalstresses severaly weaken the concrete product. Also, sulfur trioxide inthe pond water causes a weak sulfurous and sulfuric acid which may reactwith the ferric elements in the pond to form marcasite (FeS₂). Thelatter then becomes another source of sulfur in the aggregate which canalso react with the lime.

The presence of iron pyrite is particularly deleterious. Pyrites (FeS₂)are found naturally in coal and may comprise a fraction of 1 percent upto as high as 7 percent or more of the coal by weight. 2 percent ironpyrite and 10 percent ash content is not unusual in some coals, so thatif a power plant burns 10,000 tons of this coal a day, it will produce200 tons of pyrites and 1,000 tons of ash, 800 tons of which istypically fly ash and 200 tons is bottom ash. In this situation, thepond ash could contain equal amounts of iron pyrites and bottom ash byweight. Although some of the pyrite is found naturally in coal inmicroscopic or molecular form, a major portion of the pyrite content ofthe coal is granular, that is, in the form of discrete nodules whichexceed 200-mesh. Many of these nodules appear to be about the size ofsmall lead shot (-3/8) on up to 1 inch or more in diameter. Pyritenodules are exceedingly hard (hardness 6-6.5) and cannot be crushed bythe pulverizing rollers, and are relatively heavy (sp. gr. 4.8 to 5.2).They are thus separated at the pulverizer and are commonly, but notalways, recombined with the bottom ash by sluicing into the pond. Thesmall percentage of iron pyrite which is carried by the pulverized coalinto the furnace (as well as the organic sulfur content) is oxidizedwhen burned. The pyrites are converted into ferric oxide and sulfurdioxide gas. The resulting ferric oxide content in the bottom ash hasbeen found to be generally unobjectionable.

The iron pyrites, themselves, are not thought to contributesignificantly to aggregate growth as this form of mineral is reasonablystable and has little reaction with lime even when subjected to theexothermic heat of concrete setting up. Concrete blocks have been madecontaining iron pyrites which fully meet the strength requirements.However, I have discovered that a bottom ash lightweight aggregate whichcontains in excess of 1 percent iron pyrite may technically meet theASTM specifications for lightweight aggregate including the stainingindex of ASTM:C641 and yet be wholly unsuitable as a lightweightaggregate. When concrete products made with more than 1 percent ironpyrite are subjected to wetting and accelerated drying, I have foundthat distinct rust stains appear on the surface of the concrete productrendering it useless for exposed use, and I have found that thisstaining or rusting occurred even though the aggregate produced only a"light stain" index when tested according to ASTM:C641.

I have found in dry bottom installations, that if the bottom ash iscollected immediately from the bottom of the furnace, prior to beingsent to the sluicing pond, it will be essentially free of iron pyriteand sulfur trioxide. Also, due to the natural convective separationwithin the furnace it will be essentially free of excess fines so thatno more than 25 percent thereof will pass a 100-mesh screen. Thereafter,a classification and/or crushing operation may be employed to providethe required size distribution, generally eliminating the +3/8 inchmaterial. The resulting aggregate is lightweight, that is less than65-70 lbs. per cubic foot, is inert and non-reactive, and isnon-staining, even though it may have as much as 5 to 25 percent ironoxide content and is fully suitable for use as a lightweight aggregateas defined above. Also, it will be almost totally free of iron pyritesince the nodules have been separated at the crusher and have not beenrecombined as a waste product. It is particularly important that onlybottom ash be collected, and that the boiler tube slag or ash which isstripped from time to time by steam or water jets, and which is known tocontain a high percentage of sulfur trioxide, be diverted. AlthoughASTM:C618 specifies a maximum sulfur trioxide content of 5 percent (forfly ash pozzolan) I consider it to be an absolute maximum for alightweight aggregate and through experience I have found a negligableamount of free sulfur trioxide when the aggregate is collected and/orprocessed according to my invention.

I also recognize that the huge quantities of pond ash presentlyavailable can be salvaged and converted at a low cost into a suitablelightweight aggregate. Generally, this consists of the simple hydraulicseparation or classification of the heavier iron pyrite and marcasitefrom the ash. If a high natural sulfur trioxide content is also found,in excess of 5 percent, the ash cannot be used. But if it has beenallowed to settle in water for a period of time or otherwise subjectedto washing, the sulfur trioxide content will become negligible and willnot adversely affect the aggregate. After hydraulic separation of thepyrites, the aggregate can be crushed if necessary and screened toprovide the desired gradation. If the fly ash has also been diverted inthe pond, it can also be removed by washing and/or classifying, toreduce the fines to a maximum of 25 percent passing a 100-mesh screen.

It is accordingly an object of my invention to provide an improvedlightweight aggregate in which a major portion thereof consists ofbottom ash from dry bottom pulverized coal burning furnaces containingnot more than 5 percent sulfur trioxide and less than 1 percent ironpyrite.

Another object of my invention is that of a concrete product whichincorporates a lightweight aggregate as defined above.

A still further object of my invention consists of the method ofproducing a lightweight aggregate useful in making a concrete productwhich consists of the pulverizing of coal and the separation therefromof the iron pyrites, the burning of such coal in suspended state in adry bottom furnace to collect the heavier ash particles at the bottomthereof, and the crushing and grading of the collected bottom ash toremove therefrom the fraction which substantially exceeds 3/8 inch.

A further object of my invention consists of the reclamation of pond ashto provide a useful lightweight aggregate for concrete mixes andproducts.

These and other objects and advantages of the invention will be apparentfrom the following description, the accompanying drawings, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a dry bottom furnaceinstallation in an electric power plant which produces the dry bottomash used in my invention;

FIG. 2 is a plan view of the lightweight aggregate dry processing plantfor use with the installation of FIG. 1;

FIG. 3 is an elevational view of the dry processing plant of FIG. 2;

FIG. 4 is an elevational view of a wet processing plant for recovery ofa lightweight aggregate from pond ash;

FIG. 5 is a plan view of the plant of FIG. 4;

FIG. 6 is a sectional view looking transverse to the direction of mainflow of a hydraulic separator for use in the wet processing plant ofFIGS. 4 and 5; and

FIG. 7 is a side view of the hydraulic separator of FIG. 6 showing thesand screw and the pyrite pile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified diagram of a furnace and boiler of the dry-bottomtype which burns pulverized coal, in which the furnace itself is shownat 10. The coal is retained in a bunker 12 and delivered by a coalfeeder 14 to a pulverizer 15 where the coal is reduced so thatapproximately 70-80 percent thereof passes through a 200-mesh screen. Asource of hot air is delivered by a duct 16 to the pulverizer 15. Thepulverizer 15 may be of any of a number of types of which are in generalcommercial use in electric power plants, typical examples of which arethe Babcock and Wilcox type MPS Mill and the Combustion Engineering"Raymond" roller mill. A full description of the former may be found in"Steam, Its Generation and Use" (38th Ed., 1972) published by Babcockand Wilcox, 161 East 42nd Street, New York, N.Y. 10017, and the latteris described in Lorenze, "Combustion Engineering", (1951) published byCombustion Engineering and Superheater, Inc., 200 Madison Avenue, NewYork, N.Y.

The coal which has been pulverized to a fine powder in the pulverizer 15is delivered by an exhaust fan 20 through ducting 22 to a vertical stackof tangential burners 25. Secondary air to support combustion is appliedto the tangential burners 25 through a duct 26. The burners 25 arelocated at approximately 1/4 of the height of the furnace 10, and arethus spaced well above the tapered furnace bottom 30. The combustion ofthe powdered coal is almost immediate and takes place in a state ofsuspension. As previously noted, the ash content of the coal consists toa large extent of fly ash, and in a typical dry bottom installation,70-80 percent of the fly ash will be carried out of the furnace past theboiler tubes by the convective air current and will be removed bysuitable mechanical and/or electrostatic collector apparatus. Suchapparatus is diagrammatically illustrated at 32.

The heavier ash particles and the particles which have agglomerated andhave become heavier, fall to the dry bottom at the grates 40 where theyfall through the grates into a water filled ash pit 42 for collection.Also, a certain amount of slag runs down the walls of the furnace butthis slag, by the time this reaches the grates 40, has generally cooled,and a portion of the slag will also find its way into the pit 42.

The pulverizer 15 provides means for separating iron pyrites, trampiron, and other foreign material too heavy to be lifted by theconvection air in the pulverizer. In the case of crushed coal, thistrash material generally consists of iron pyrite which, of course, issubstantially heavier than coal and which generally is too hard to becrushed or ground by the rollers. In a conventional furnaceinstallation, the pyrites are expelled from the pulverizer and collectedin a pyrite box 45 for disposal. The small fraction of iron pyrite orrelated ferric materials which is carried with the pulverized coal isoxidized in the heat of the furnace.

Furnace installations of this type are generally provided withsluiceways formed within the concrete floor of the power plant. In FIG.1, there is illustrated a typical sluiceway 50 which runs beneath theash pit 42 and the pyrite box 45. A supply of water is provided to thesluiceway, and the bottom ashes from the pit 42 are first run through apair of coarse crushing rollers merely to reduce the size of the largerslag and/or ash particles so that the same can be handled in thesluiceway, usually taken to 11/2 inches in diameter, and from there theash is disposed of either on a continuous or intermittent basis into thesluiceway 50. The sluiceway 50 leads to a remote disposal pool.

The pyrites in the box 45, on the other hand, are generally accumulatedand are dumped into the sluiceway 50 on an intermittent basis andcarried by water to the pond. Similarly, the boiler tube ash whichaccumulates on the boiler tubes themselves is stripped off from time totime with steam or water jets, or acid, usually when the furnace is shutdown and this ash, which may contain a high percentage of sulfurtrioxide is also dumped into the sluiceway 50 for disposal out of thepower plant and into the pond. Also, the collected fly ash may be putinto storage bins. The economizer ash (a coarser form of fly ash) isalso temporarily stored. Usually an electric power plant produces anexcess fly ash over which it can sell or otherwise gainfully dispose of,and the excess ash is commonly sluiced to the disposal pond.

FIGS. 2 and 3 show a dry processing plant by means of which I salvageand separate the dry bottom ash from the deleterious substances producedby the coal burning installation of FIG. 1. The sluiceway 50 terminatesin an individual sluice pipe 52 leading to the ash disposal pond 54 asshown in FIG. 2. Four disposal pipes 52 are shown corresponding to fourfurnace-boiler installations. Each pipe 52 is provided withsolenoid-operated diverter valves 55 by means of which the controller inthe power plant can divert the effluent from the pipe 52 into the pond54. When the diverter valve 55 is closed, the effluent is disposedthrough feed pipes 60 in one of a pair of side-by-side holding bays 58.The back walls 61 of the bays are provided with drain openings 62through which the excess water may flow back into the pond 54. Theholding bays 58 are arranged so that while one is being filled the otheris draining. The bays are open at the front, as shown, to permit accessthereby by suitable loading apparatus. A second diverter valve 65 isprovided in the line 60 so that the power plant operator can selectwhich of the holding bays 58 is to be filled. Thus, during night andweekend operations, the diverter valve 65 may be operated to assure thatone or the other of the bays 58 will be filled with bottom ash.

The diverter valves 55 permit the power plant operator to divertdeleterious substances. Thus, these valves will be operated to divertthe iron pyrites which have been collected by the box 45 when these arebeing sluiced out of the plant to assure that none of the iron pyritesfind their way into the bays 58. Similarly, during boiler tube cleaningoperations, during shutdown of the plant, and during fly ash disposal,the valves 55 are operated to assure that these materials are divertedto the pond 54. In this manner, only bottom ash from the pit 42 iscollected and accumulated in the bays 58. This bottom ash is essentiallytotally free of iron pyrites, excess fines, and excessive sulfates.

I have found that dry bottom ash collected as described as describedabove directly from a dry bottom coal burning furnace is entirelysuitable as a lightweight aggregate for concrete mixtures. If it isprevented from entering the pond, it will have substantially no sulfurtrioxide or other sulfate content. Further, it will have a gradation,with simple processing and blending, if necessary, and a dry weight,such as to meet the ASTM specifications for such an aggregate.Advantageously, the convective selection in the furnace, itself, hasremoved from the ash content of the pulverized coal a substantialportion of the fines so that in the bottom ash recovered by thisinvention no more than 25 percent of this material will pass a 100-meshscreen, and typically, the 100-mesh material will be 10 percent of thedry bottom ash. The following is an analysis by typical ranges ofconstituents in dry bottom ash:

    Al.sub.2 O.sub. 3                                                                        = 15-35%      FeS.sub.2  = 0-1%                                    SiO.sub.2  = 40-55%      MgO = 1-3%                                           Fe.sub.2 O.sub. 3                                                                        = 5-25%       CaO = 1-5%                                           SO.sub.3   = 0-5%        TiO.sub.2  = 1-3%                                    C = 0-5%                                                                  

In FIGS. 2 and 3 I have shown a dry processing plant for sizing andclassifying the ash from the holding bays 58. The drained material stockpiled within the bays may be loaded out by any suitable front end loader68 and carried to the hopper 70 of a belt feeder 72 where it is carriedby a conveyor 74 to a classifier 76 such as a vibrating screen.Typically, the vibrating screen will separate the +3/8 inch material andwill pass the -3/8 inch material. The +3/8 inch material may bedelivered on a belt 78 directly to a stock pile 80, or may be appliedthrough a pair of crushing rollers 82 which are adjusted to reduce thismaterial to 3/16 inch and then stock piled as indicated at 84.

The -3/8 inch material passing the vibrating screen 76 is carried fromthe classifier and is stock piled by a conveyor 86 as indicated at 90.The pile 90 will thus contain material -3/8 inch down to dust andgenerally qualifies without further gradation as a "combined fine andcoarse aggregate -- 3/8 inch to 0 inch" as specified in ASTM:C330, 331,and 332 and will have a dry loose weight of approximately 65 pounds percubic ft. or less. In some cases, the dry loose weight may somewhatexceed 65 pounds but will not exceed 70 pounds per cubic ft. In thiscase, the material in the pile 90 can usually be qualified withoutsubstantial further gradation as a "fine aggregate" in accordance withthe above-identified specifications.

Suitable gradations in the material taken from the pile 90 may be easilymade by blending. For example, a material may be taken from the pile 84and blended to create a somewhat lighter aggregate with a sizedistribution falling within the range defined for a "combined fine andcoarse aggregate." Also, fly ash may be blended into the mix if a highpercentage of fines is desired, such as where a smoother texturedconcrete product is desired. The material in stock pile 80 may also besized to eliminate that which exceeds 1 inch and used as a "coarseaggregate" in structural concrete mixes, or may be used for fill, roadbase stabilization, or the like.

As noted above, my invention also resides in the utilization of pond ashitself, which pond ash may contain substantial quantities of iron pyritenodules, and fines in the form of fly ash, which have been dumped orsluiced into a disposal pond. The deleterious sulfur trioxide and othersulfates are generally water soluble, and thus may be eliminated as afactor by the employment of the wet processing plant shown in FIGS. 4and 5. Here the pond 95 is fed by one or more lines 96 leading from thepower plant. A typical power plant may use up to 250,000 gallons ofwater to sluice 20 tons of ash to the pond. The pond ash can be removedfrom the pond while the pond is in active use by any suitable means, andfor this purpose I have illustrated a pontoon-mounted dredge 100. Pondash and water are conveyed by means of a pipe 102 to the inlet of one ormore hydrocyclones 105. The purpose of the hydrocyclones is to removethe excess pond water, and a portion of the fine material. Typically,3000-6000 gallons of water will be removed by the dredge 100 in removing5-10 tons of pond ash. The excess water and a portion of the finesremoved by the hydrocyclones 105 may be returned to the pond 95.

At this point, I find it advantageous to remove the larger fraction suchas +3/8 inch, by means of a vibrating screen 110. Experience has shownthat a substantial portion, often in excess of 50 percent, of the ironpyrite content of the pond ash comprises material which will not pass a3/8 inch screen and therefore removal of the +3/8 inch material will, atthe same time, result in an elimination of a substantial portion of thepyrite content. The +3/8 inch material from the top of the screen 110 isconveyed laterally to the pile 112 by a conveyor 113 as shown in FIG. 5.

I now apply the -3/8 inch material to a suitable hydraulic classifier115 to separate the heavy particles in excess of 4.0 specific gravityfrom the pond ash. Pyrite and marcasite have a specific gravity inexcess of 4.5, in the range of 4.8 to 5.2. A suitable hydraulicclassifier for this purpose is described in U.S. Pat. No. 3,385,432.

In FIGS. 6 and 7, there is shown a simpler form of underwater classifier115 which is particularly useful in the separation of the substantiallyheavier pyrite components from the bottom ash. The -3/8 inch materialfrom the screen 110 is passed through a conduit 120 and into a box 125.The bottom of the box is provided with a 3/8 inch wire mesh screen 126.A hopper 127 beneath the screen leads downwardly to the inlet of anupwardly inclined sand screw 130. The entire box 125 as well as a majorportion of the length of the screw 130 operate under water as shown bythe water line 128 in FIG. 7. The heavier materials passing from theconduit 120 into the box 125 fall through the screen 126 and into thehopper downwardly to the entrance of the screw 130 where they areremoved continuously by the screw conveyor to a pyrite pile 132. Eventhough some of the lighter fraction may also pass through the screen125, this lighter fraction will be displaced by the heavier pyrites, andin effect, will float to the top and be carried through the box 125 withthe remaining pond ash material.

The lower end of the conduit 120 leads to a further hydraulic classifierto remove the remaining fine material in excess of 25 percent. I use aweir box 140. The weir box 140 is provided with an overflowing weir wall142, and the overflowing product is lead by a pipe 143 to disposal,which may be back into the pond. The weir box 140 functions to separatethe lighter and finer fraction consisting of the extreme fines whichgenerally are less than 100-mesh in size, to remove this excess of finesfrom the pond ash material. This apparatus is particularly useful wherethe pond ash includes a mixture of fly ash from the mechanical orelectrostatic precipitators.

The remaining material is carried by a second sand screw 144 from thebox 140 to de-water the material and present it onto the surface of aconveyor 145 for depositing in a pile 150. The pile 150 will now containa useful lightweight aggregate with a size distribution of -3/8 inchdown to 100-mesh size and smaller. It is free of pyrites and due to thewashing provided by the excess water and de-watering which occurs at thescreen 110 and the screw 144, this material is also substantially freeof SO₃. If further fines are required in the aggregate, such as may bedesired from time to time to provide a finer texture to the concreteproduct, the weir 142 may be approximately adjusted to retain more ofthe fine material, or alternatively, fly ash may be blended with thematerial of the pile 150 to achieve the desired balance offine-to-coarse material.

In order to demonstrate the usefullness of dry bottom ash as alightweight aggregate in the manufacture of concrete products, a testwas undertaken in which thirteen masonry units of the type described inASTM:C-90 were made in accordance with each of the four mix designs asidentified below in Table I.

                                      Table I                                     __________________________________________________________________________                    Control                                                                              Mix No. 1                                                                          Mix No. 2                                                                           Mix No. 3                                   __________________________________________________________________________    Cement           575   575   575   575                                        Fly Ash          175   175   175   175                                        Bottom Ash      --    2400  3400  3900                                        Fine Haydite    2400  --    --    --                                          Coarse Haydite  1000  1000  --    --                                          Sand            1000  1000  1000   500                                        Plasticizer (oz.)                                                                              22    22    22    22                                         Blocks per Batch                                                                              168-183                                                                              164   148   155                                        1 day strength ave.*                                                                          1360  1300  1240  1215                                        3 day strength ave.                                                                           1340  1355  1335  1155                                        7 day strength ave.                                                                           1410  1380  1430  1330                                        28 day strength ave.                                                                          1340  1460  1550  1430                                        Absorption (ave.) lbs.                                                        per cubic foot  12.7   10.6  11.2  12.2                                       Specification -- ASTM:C-90                                                                    18 max.                                                                             18 max.                                                                             15 max.                                                                             15 max.                                     Unit Weight (ave.) Lbs.                                                                       91.9  101.4 109.0 107.1                                       per cubit foot                                                                Specification -- ASTM:C-90                                                                    85-104                                                                              85- 104                                                                             105- 124                                                                            105-124                                     Ave. Weight of Block (lbs.)                                                                    27.95                                                                               30.38                                                                               33.28                                                                               32.64                                      __________________________________________________________________________     *ASTM:C90 states that the unit must have 1000 PSI strength at time of         delivery for grade N blocks.                                             

In the "control" mix, the cement blocks were made with a commercial,lightweight expanded shale aggregate sold under the trademark "Haydite",a product of Hydraulic Press Brick Company, Cleveland, Ohio 44131. Thefly ash constituent in each of the four mixes was added as a pozzolan.The bottom ash constituent of mixes 1, 2 and 3 was taken from thedisposal pond of the Stuart Station of the Dayton Power and LightCompany of Aberdeen, Ohio. The pond ash was screened on a 3/8 inchscreen and used without gradation or classification and withoutseparation of the pyrites. It contained no fly ash. A sample of theaggregate used in mixes 1, 2 and 3 was tested to determine compliancewith ASTM:C-331-69 for grading, unit weight, deleterious substances, andconcrete-making properties. The results of this test are set forth inTable II.

                                      Table II                                    __________________________________________________________________________                             ASTM:                                                Item        Bottom Ash   Specification -- C-331                               __________________________________________________________________________    Oven Dry Weight                                                                           67.3 lbs per c.f.                                                                          70 lbs. per c.f. max.                                Organic Impurities                                                                        Clear        Clear                                                Staining Index                                                                            30           80                                                   Clay Lumps  none         2.0 per cent max.                                    Loss on Ignition                                                                          5.7 percent* 5 percent max.                                       Friable Particles                                                                         9.1 percent  none stated                                          Gradation (Dry Sieve)                                                         Sieve Size 3/8                                                                              4     8                                                                              16  30                                                                              50  100                                                                              200                                         Bottom Ash 100                                                                              82*  60                                                                              41  28                                                                              19   10                                                                               4                                          Specifications for                                                                       100                                                                               85-100                                                                              40-80 10-25                                              "Fine Aggregates"                                                             __________________________________________________________________________

It will be noted in reference to Table II that the lightweight drybottom ash exceeded the gradation test for a "fine aggregate" only inthat an excess of 3 percent was retained on the No. 4 screen and thatthe Loss on Ignition exceeded the specification by .07 percent. It metthe ASTM gradations for a "combined fine and coarse" aggregate.

The masonry units of each of the four mixes were tested for absorptionin accordance with ASTM:C-90-70, and the results of Table I indicatedthat each of the three mixes in which the bottom ash was used as thelightweight aggregate met the requirements of these specifications.

It should be noted in Table II that the bottom ash used in mixes 1, 2and 3, when subjected to the staining test of ASTM:C641 produced only a"light stain" index of 30 even though it was later determined that thisaggregate contained an unsuitable quantity of iron pyrite, substantiallyin excess of 1% by weight. The deleterious pyrite content was noted whentwo units of each of the four mixes were subjected to an acceleratedstaining test. In this test, the eight units involved were subjected tofour cycles consisting of oven drying at 212°-239° F for eight hours,cooling for one hour, and soaking in clear water at room temperature for15 hours. The specimens from mixes 1, 2 and 3 exhibit numerous ruststains on all surfaces while the specimens from the control mix did notexhibit any stain.

A further specimen of dry bottom ash was then obtained directly from thebottom of furnace No. 3 at the previously defined Stuart Station, whichis a dry bottom furnace. This was made into a mix consisting of 2,000grams of dry bottom ash, 300 grams of type 1A cement and sufficientwater to hold the mix together which was tamped into two 8 × 8 × 11/2inch pans and cured in a moist oven for 24 hours. The resulting castingswere subjected to 25 cycles of the previously defined acceleratedstaining test, and no visible stain was produced. Further, a specimen ofthe aggregate employed was subjected to ASTM:C-641 and produced a stainindex of 10 which is less than "very light stain". Also, a further testusing this dry bottom ash was tested for pop-out materials according toASTM:C-151, "Test for Autoclave Expansion of Portland Cement." Nopop-outs were detected.

A 100-day shrinkage test was run in accordance with ASTM:C-330 andASTM:C-157 providing a shrinkage of 0.076%. The allowable shrinkage is0.10%.

A further concrete mix using the dry bottom ash of the precedingparagraph was made for the purpose of performing twenty-eight daystrength tests in accordance with ASTM:C-330. Concrete test cylinderswere made in accordance with the following mix design:Cement 470 lbs.FlyAsh 140 lbs.Sand 1200 lbs.Bottom Ash 900 lbs.Water 300 lbs.Water Reducer3 oz. per 100 lbs. of cement and fly ashAir Entraining Agent 1 oz. per100 lbs. of cement and fly ashUnit Weight ofconcrete in plasticform119.8 lbs. per cubic ft.Bulk Dry UnitWeight (ASTM:C-567-71) 106.9 lbs.per cubic ft.The average strengths were determined to be asfollows:3-day strength 1060 PSI and 1110 PSI7-day strength 1500 PSI and1520 PSI28-day strength 2600 PSI and 2650 PSI

The above-identified test shows that a concrete mix using dry bottom ashas a lightweight aggregate exceeded ASTM:C-330 specifications forstructural concrete. Also, the mix fully met the requirements of TheManual of Concrete Practice, Part I, Sections 213-7, paragraphs 1.5"Definition of Light Weight Aggregate Concrete" published by theAmerican Concrete Institute, P.O. Box 4754, Readford Station, Detroit,Mich. 48219, in which are recommended a 28-day compression strength of2500 PSI (minimum) and a dry unit weight of 115 lbs. per cubic ft.(maximum).

It will accordingly be seen that this invention provides a newlightweight aggregate which consists of essentially bottom ash derivedfrom a dry bottom pulverized coal burning furnace. This aggregate isnon-reactive as it contains no more than 5 percent sulfur trioxide andis essentially non-staining in that it contains less than 1 percent ironpyrite. The classification which occurs within the furnace itself, inthe case of the embodiment of FIGS. 2 and 3, assures that no more than25 percent of the resulting product will pass a 100-mesh screen.Preferably, this percentage is approximately 10 percent. In the case ofthe embodiment of FIGS. 4 and 5, the weir box 140 assures theelimination of the excess fines in the event that the pond ash may becontaminated by the discharge of fly ash therein from the precipitators.

In the case of the embodiment of FIGS. 2 and 3, the product isessentially free of iron pyrite as the nodules have been separated atthe pulverizer, and the arrangement of the diverter valves 55 assuresthat they do not contaminate the material in the bays 58. However, wherethe material in the existing pond is reclaimed, the hydraulicclassification apparatus used in this connection in the embodiment ofFIGS. 4 and 5 removes the pyrites and other heavy particles from theproducts.

Basic concrete mixes for poured structural concrete as well as mixes forconcrete masonry units may vary widely in the relative amounts of sand,cement, fly ash as a pozzolan, aggregate, and water. Generally,increasing the percentage of lightweight aggregate decreases the unitweight of the resulting concrete product but increases the cost of theproduct over that where only sand, gravel, or stone is used as theaggregate. Thus, mixes for masonry units (blocks) may have from 8-20percent cement, 10-50 percent sand, and 25-90 percent lightweightaggregate. Mixes for structural concrete may have 8-20 percent cement,10-50 percent sand, and 10-50 percent lightweight aggregate. In each,fly ash may also be used as a pozzolan. The dry bottom ash recovered andprocessed according to the methods of my invention may consist of themajor portion of the lightweight aggregate used in such mixes, and issuitable for use as the entire lightweight aggregate of such mixes.

Also, while my invention is particularly useful as an aggregate inconcrete block mixes with a maximum particle size of approximately 3/8inch, my invention is not limited to this particular size gradation, andcoarser material may often be used for specific applications and thismaterial which does not exceed 1 inch may qualify as a "coarseaggregate" under the ASTM C-330.

While the processes and products herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise processes and products, and that changesmay be made therein without departing from the scope of the invention.

What is claimed is:
 1. A lightweight concrete product having as itsmajor ingredients a pozzolanic material including primarily Portlandcement; sand; and a lightweight inorganic non-reactive aggregate formedof dry bottom ash, said major ingredients having the followingapproximate proportions:

    pozzolanic material                                                            8 to 20 per cent                                                             sand                                                                          10 to 50 per cent                                                             aggregate                                                                     remainder                                                                 

said bottom ash aggregate consisting of dry bottom ash essentially asformed as the waste by-product of burning pulverized coal in a drybottom furnace, said dry bottom ash containing no more than 5 per centsulfur trioxide and less than 1 per cent iron pyrite by weight.
 2. Theproduct of claim 1 consisting of hollow load-bearing masonry units. 3.The product of claim 1 in which said bottom ash has a size such thatapproximately all passes a 3/8 inch screen with not more than 25 percent passing a 100-mesh screen.
 4. A concrete mix for making lightweightconcrete masonry units, structural concrete or insulating concrete,having as its major ingredients a pozzolanic material includingprimarily Portland cement; sand; and a lightweight inorganicnon-reactive aggregate formed of dry bottom ash, said major ingredientshaving the following approximate proportions:pozzolanic material 8 to 20per centsand10 to 50 per centaggregateremaindersaid bottom ash aggregateconsisting of dry bottom ash formed as a residue in dry bottompulverized coal burning furnaces, said bottom ash being free of excessfines so that no more than 25 per cent thereof is less than 100-meshsize, and containing no more than 5 per cent sulfur trioxide and lessthan 1 per cent iron pyrite by weight.
 5. The concrete mix of claim 4particularly suited for making masonry units in which said dry bottomash has a maximum particle size of approximately 3/8 inch.
 6. Theconcrete mix of claim 4 particularly suited for making structuralconcrete in which at least a fraction of said dry bottom ash exceeds 3/8inch but does not exceed 1 inch.
 7. A concrete mix for lightweightconcrete products having as its major ingredients a pozzolanic materialincluding primarily Portland cement; sand; and a lightweight inorganicnon-reactive aggregate formed of dry bottom ash, said major ingredientshaving the following approximate proportions:pozzolanic material 8 to 20per centsand10 to 50 per centaggregateremaindersaid bottom ash aggregateconsisting of bottom ash essentially as formed as the waste by-productof burning pulverized coal in a dry bottom furnace, said bottom ashbeing essentially free of fly ash and excess fines to have a size suchthat not more than 25 per cent thereof passes a 100-mesh screen.